Factoring GHG Emissons into Optioneering: 90% Reduction for Cambridge WWTP

Derek Anderson (PDP Ltd.), Yashika De Costa (PDP Ltd.), and Daniel Garden (PDP Ltd.)

The Paris Agreement on Climate Change catalysed legislative and policy changes across New Zealand, including the target of achieving Net Zero carbon emissions by 2050. Achieving this target requires engineering design to consider Greenhouse Gas (GHG) emissions in the optioneering phase for all infrastructure projects. 

Whilst there are a number of methodologies available, estimating GHG emissions from Wastewater Treatment Plants (WWTPs) has a degree of uncertainty, and reducing this uncertainty requires onsite measurement to calibrate and refine current models and emissions factors. As international and New Zealand WWTPs verify and report their respective emissions factors for various wastewater treatment unit processes, the accuracy of predictive models will continue to improve. However, planning and design of WWTP upgrades or new WWTPs, requires designers to draw on existing data and models to inform engineering decision making.

Minimising GHG emissions was identified as a key criterion for the new Cambridge WWTP, along with other environmental, social, cultural, and technical criteria. In order to estimate GHG emissions for the shortlisted advanced Biological Nutrient Removal (BNR) treatment options being considered, an amalgamation of assessment methodologies was necessary. Methodologies included Water New Zealand’s Carbon Accounting Guidelines for Wastewater Treatment: CH4 and N2O, the 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories, the Ministry for the Environment’s guidelines on measuring emissions, and outputs from the WWTP modelling software EnviroSim BioWin.

This paper outlines the operational GHG emissions estimate approach undertaken during the design optioneering phase of the Cambridge WWTP upgrade, including insights attainable via amalgamating these different methodologies. Additionally, this paper highlights the significant reduction in operational GHG emissions attainable for municipal WWTPs, via shifting from an anaerobic and aerated lagoon-based treatment process to a 4-stage Bardenpho Membrane Bioreactor (MBR) treatment process. Finally, the criticality of further research and monitoring to verify existing emissions guidelines and model parameters is stressed.

Notably, shifting from the status quo of an anaerobic and aerated lagoon-based treatment to the selected MBR treatment process was estimated to reduce the annual operational GHG emissions by approximately 90%. Further emissions reductions are likely with a proposed 1.5 ha solar array onsite. This significant reduction clearly aligned with Waipā District Council’s overarching carbon reduction strategy and supported the “best for awa” approach to the long-term resource consenting of the Cambridge WWTP upgrade and the Waikato River receiving environment.